This invention relates to a bipolar static semiconductor memory device, particularly to a semiconductor memory having improved protection for the stored contents.
To increase memory capacity, it is attempted to form a memory device by combining a plurality of semiconductor memory chips.
In this case, although a memory device provides a large capacity, only one chip is selected and a memory cell in such a selected chip is written into or read from. Other chips and memory cells are not selected. However, whether a chip is selected or not selected, an address is still applied to each chip, and the word line of a non-selected chip, corresponding to the selected word line of selected chip, becomes a high, H level. At this time, therefore, if noise occurs on the bit line, a erroneous writing may sometimes occur. Namely, in the case of the conventional static memory where each memory cell comprises a flip-flop circuit, the bit line voltage of a non-selected chip has been the same as the bit line voltage of the selected chip in the read operation and therefore the information in the cells of the non-selected chip is also read to the bit line.
At this timing, if a negative noise pulse appears on the bit line the transistor in the OFF condition, of the flip-flop type memory cell, turns ON. As a result, the transistor in the ON side may turn OFF, and the stored information in the cells may be destroyed.
Such noise may result from the noise of power supply line or signal lines due to, for example, a variation of input data and address signal appears on the bit lines.
For this reason, it is essential to prevent destruction of information stored in the cells of such a non-selected chip.